It is well established that the incorporation of certain types of carbon nanofibers into polymeric materials can impart electrical conductivity to such materials that are generally regarded as insulators. Carbon nanofibers can be dispersed in a polymer by various well-known techniques such as melting and kneading to form an admixture that can be subsequently shaped to form an electrically conductive article. The use of a conductive fiber is highly desirable since a given weight of such a material generates a large number of contact points within a polymer matrix. The widespread interest in electrically conductive polymers is stimulated by the possibility that such materials could have utility in semiconductor chips, integrated circuits, lightweight battery components, sensors, electro-chromic displays, anti-static coatings, static dissipation, fuel hoses, connectors and packaging items.
Various types of carbon nanofibers have been suggested as being suitable for use as conductive filler components for polymeric materials. For example, U.S. Pat. Nos. 4,663,230, 5,618,875 and 5,908,585 teach of the use of incorporating fibrils and carbon filaments (also known as carbon nanofibers) into polymeric matrices to render the composite materials electrically conductive. Chemical, physical and electrical advantages resulting from chemical functionalization of the surfaces of carbon nanotubes prior to imbedding the nanofibers into a polymer matrix are disclosed in U.S. patent application Ser. No.20030089893.
While there are many current commercial applications for conductive polymers in the form of films and sheets, a need exists to develop such materials in the form of a fiber. Thermoplastics such as nylon and polyester that contain traditional particulate conductive fillers such as carbon black, metal or metal oxide powders are difficult to extrude and draw into a fiber. The ability to decrease the percent weight of filler in the resulting polymer fiber without sacrificing the electrical conductive properties can be achieved by substituting carbon nanofibers for the traditional powdered additive. Because of their high aspect ratio (length/diameter) of about 100 to 1000, carbon nanofibers contact each other over a much larger surface area than the spherically shaped conventional powdered particles for a given loading. Such a material, however, suffers from the fact that while the bulk properties of the polymer exhibit high electrical conductivity, the poor contact of the conductive component with the exposed polymer surface results in a significantly lower electrical conductivity in those regions. Measurements performed on polymer composites containing metal coated carbon fibers indicate that the surface conductivity is between 103 to 105 times lower than that of the bulk conductivity (“Conductive Polymers and Plastics in Industrial Applications” {Larry Ruppercht Editor} Society of Plastics Engineers page 149, (1999). Consequently, there is a need in the art for polymer structures, including both sheets and fibers that have both high bulk conductivity and high surface conductivity.